Spatial luminous intensity distribution, which can be accurately measured by goniophotometers, is an important parameter for lamps or luminaries. A photometer head can be rotated around a test light source to measure the luminous intensity of the light source in different spatial angles. The luminous intensity is usually determined by the measurement results of illuminance and the photometric distance law.
The goniophotometers with mirrors are recommended by the International Commission on Illumination (CIE) and other international standards, because they help measure luminous intensity distribution in a longer measurement distance and also help make a test light source stable. There are generally two kinds of existing goniophotometers with mirrors: 1) centric moving mirror goniophotometer 100 as shown in FIG. 1, wherein a centric moving mirror 102 is at a rotation center, and can be rotated around a main horizontal axis 108 during the measurement. A test light source 104 holding by a lamp support arm can be rotated around the centric mirror 102, and the lamp support arm can be synchronously rotated in the opposite orientation around a horizontal auxiliary axis 110 so as to keep a burning attitude of the test light source 104. The test light source 114 can also be rotated around a vertical axis 112. The centric mirror 102 reflects a light beam from the test light source 104 to a photometer 106. 2) circus moving mirror goniophotometer 200 as shown in FIG. 2, wherein a test light source 204 is at a rotation center. A mirror 202 can be rotated around the test light source 204 and reflects a light beam to a detector 206. There are two rotation axes in the circus moving mirror goniophotometer 200: a main horizontal axis 208 and a vertical axis 210 of the test light source 204.
Although they are relatively accurate in measuring luminous intensity distribution, the above two kinds of goniophotometers have disadvantages. With respect to the centric moving mirror goniophotometer 100, firstly, the test light source 104 needs to move in a large space, thus, it is almost impossible to avoid vibration and shocking of the test light source 104, and consequently the light source 104 can not work very stably because of existence of centripetal force and air flow. Secondly, because of the synchronous rotation between the main axis and the auxiliary axis, it is very difficult for the centric moving mirror goniophotometers to achieve high angle accuracy. In addition, this kind of goniophotometers have a very high darkroom, thus, the total cost is largely increased.
Although the circus moving mirror goniophotometer 200 can overcome the problem of low stability of the test light source as with the centric moving mirror goniophotometer 100, they have following problems. First, the incident angle of a principle light to be measured is strictly limited to no more than 2.5° to a detector by CIE test report No. 70-1987, The Measurement of Absolute Luminous Intensity Distribution. This requires a very long measurement distance and thus a very long dark room. Also, it will be very difficult to measure a light with low intensity in such a long distance. Secondly, the circus moving mirror goniophotometer 200 is required to have larger view angles, and, when the detector has a large view angle, it is very difficult to prevent stray light from entering the detector.
In addition, there are few existing goniophotometers that can measure luminance of different points or luminance distribution of the test light source in different spatial angles, despite of the importance of the luminance parameters for inspection and application of the light source. For example, luminance parameters are necessary for evaluation of glare in a lighting design. However, glare parameters are usually calculated by average luminance of the luminaries obtained as the quotient of measured luminous intensity divided by luminous area. In many cases, this is not very accurate. In addition, it cannot show an intuitionistic picture of the test light source in different view angles.